Development of RTP processes for advanced Cu2ZnSnSe4 solar cells

2019 edition

Alejandro Hernández-Martínez

This work describes the results obtained on the implementation of Rapid Thermal Processing (RTP) selenisation strategies for the fabrication of Cu2ZnSnSe4 based solar cells. This semiconductor belongs to the family of compounds known as kesterites, which are receiving a growing interest because of their potential for the development of cost-efficient sustainable PV technologies. Kesterites are compounds of the type Cu2ZnSn(S,Se)4 that show a direct band-gap with values comprised between 1.0 and 1.5 eV depending on their composition (S/(S+Se) content ratio). In contrast to PV industrial chalcopyrite technologies based on Cu(In,Ga)(S,Se)2, kesterites only contain earth-abundant elements with very low toxicity. This, together with the high level of technological compatibility with the chalcopyrite technology, has led to the identification of kesterites as the future alternative to chalcopyrites for a TeraWatt sustainable production.

The main objective of this work is the implementation of RTP processes in the baseline kesterite technology that has been developed at IREC. This includes two sequential processes: the deposition of the metallic precursors via sputtering and their next annealing treatment either with selenium, sulfur or both. This process, which is realized in conventional tubular furnaces, has allowed us to obtain photovoltaic conversion efficiencies up to 10.6%. The analysis of the transferability of these processes to an industrial scale requires the study of the implementation of RTP strategies replacing the conventional tubular furnaces, due to higher compatibility with module production lines. Also, those types of processes allow a higher control for the thermal treatments, and are better suited for the development of complex recrystallisation processes involving several steps. They are also better suited to obtain absorber with gradient band gap profile which is crucial to reach very high efficiency. Actually, the best results obtained via RTP at IREC have shown efficiencies up to 8.3%, corresponding to a RTP two-step selenisation processes performed at 400ºC and 500ºC with a heating ramp of 3ºC/s and 1ºC/s for 3 and 5 min respectively